For the last month or so, I have been hearing a whining/whirring noise directly related to engine RPM coming from the front of my Mercedes. About two weeks ago, we went to use it and the battery was dead, I let is sit a day or so (three cars) before I got around to jump starting it, it started right up, but would not run once you removed the cables. Also, the battery is so dead, it will not take a charge.

Will an alternator short so that it kills the battery when it fails? and does it sound like this was my alternator failing?

I will fix it myself, the labor guide calls for .9 hours so it has to be a pretty easy job. I am really hoping this is my alternator and NOT a wiring issue, however, ever where the wiring harness is exposed it appears like new and supple.

One other question related to the A/C, it works great, will freeze meat in the car, however, something during heavy rain it quits for a bit, this has happened twice this hear. I am at a loss here... thinking maybe something is getting wet?

Jerry

 

Before you replace the alternator a few questions.what year model and miles?this one may seem odd to ask but since you appear to be new to the diy scene.Did you pop the hood and look for the sound source?Is the belt at proper tension?worn, or frayed,or not there at all?
The car will start with a jump but not with the cables off,dead battery,check it with a multimeter.Replace the battery.Have your radio code on hand to reprogram the radio.Then start the car and check alternator output.
If that belt is so loose that it is slipping it may be slipping to the point that under load it
(a/c on ) slips on the compressor and shuts down a/c operation. You need to replace that battery first,and no you would not kill a battery ,you will drain it down with a bad alternator but if it was good it would not kill it.
Was your battery light on?That is really not for the battery,that light indicates charging status.
report back with what you see,hear and test.
You have other cars so you could put in one known good battery to get it running and test the other charging functions.Be careful around the moving belt,test tension with engine off.
have fun
ohlord

 

I might mention again, that it will not run without external voltage being applied. If you jump start it, and remove the cables the car immediately dies, this is why I am thinking alternator trouble. The belt is in pretty poor condition, frayed on one edge, however the tension seems good.

The sounds was not necessarily a failure sound. I am a fairly new owner to the car, so I had nothing to base as a comparison, however, in retrospect, after what would appear as an alternator failure, I thought I might mention the sound. I don't think the battery light ever came on, however, I was not hte last one to drive it before failure.


As for me being new to the DIY scene, I do have a bit of experience, I have even done head gaskets and timing belts... but no MB experience.

 

oh and it is a 1995 with 104K

 

Good battery with a bad alternator--car will run until the battery drains so low it cant fire anymore---that is your key that it is the battery,and not the alternator.Like i said swap in one of your other good batteries and then check operation of the other systems while running and able to observe it in person(under the hood and the battery dash light.
good luck
ohlord

 

Shouldn't the alternator keep the car running once started even if the battery is dead?

 

Hi,
You need either a good battery or a working alternator to keep the car running once you have removed the jump leads.

A possible scenario is that the battery got an internal short and put (or continues to put) high load on the alternator (hence the noise) but is not generating any voltage at the battery terminals due to the internal short. IF the alternator is still OK you need to get the dead battery off before you do any more running.
Once a battery has gone below 12.5 volts it is NO GOOD reliable even if it temporarily takes a charge.

Aklternatively the alternator stopped charging (faulty diaode rectifier etc) and this in turn caused the battery to drain.

Whatever happens you need a new battery. get a new one and after installation check voltage at battery with engine running at different speeds. Do you see circa 14volts? If not you need a replacement alternator also(or at least the voltage regulator part)

wrt the A/C. This may be shutting down due to evaporator icing caused by the high humidity.
A sensor determines whether the evaporator is icing (air off temperature should be above 0degC (circa 2 degC I think) since as it becomes clogged with ice airflow across the evaporator is reduced. If icing is detected the system dynamically gives feedback to the evaporator throttle valve which reduces the evaporative process. Sounds like your system is working (at least from a temperature off point of view) a little too well!

Alternatively maybe the systemn has been overfilled with refrigerant and system pressure is being exceeded which will shut down the compressor.

So the key question is how do you know the system has shut down? I suspect the most likely cause is the later scenario.

Stuart

 

Interesting.... good advice from both of you. I will pick up a battery today. I also have access to an A/C machine, so I will empty the system, draw a good vacuum, and refill with the proper amount and see if that doesn't fix it. I tend to think you are right on this one, when the A/C quit, I looked at the compressor and it was spinning but the clutch was not engaged, so for whatever reason the system shut down...

thanks again, I will post what I find.

 

The key again is it starts with a jump,so the battery is dead,it will run with a good battery and a bad alternator.But it won't run with a dead battery.Get a battery.
ohlord